Proper protein folding is a key issue in maintaining healthy, functional neurons. Neurodegenerative diseases such as Alzheimer's, Parkinson's, Huntington's and Lou Gehrig's disease (amyotrophic lateral sclerosis or ALS) all result from protein misfolding in neurons. Similar disruptions of proteome homeostasis cause neuronal degeneration of the photoreceptor cells of the retina. Many severe retinopathies have been linked to misfolding mutations in rhodopsin and other photoreceptor proteins. A key component of molecular chaperone system that maintains proteome homeostasis in photoreceptor cells is the cytosolic chaperonin complex (CCT, Chaperonin Containing Tailless polypeptide 1) which folds actin, tubulin and dozens of other cytosolic proteins. Among these CCT substrates are the G protein subunits (G) which form the G 1?1 and G5-RGS9 (Regulator of G protein Signaling) dimers that are essential components of the phototransduction cascade. All G subunits require CCT and the co-chaperone phosducin-like protein 1 (PhLP1) to fold G and assemble into functional dimers. Very recently, an additional role for CCT in Bardet-Biedl syndrome (BBS) has been demonstrated. BBS is a genetic disease of ciliary dysfunction displaying multiple pathological conditions including retinal degeneration. Mutations in 14 BBS proteins have been associated with the disease. Seven of them (BBS 1-2, 4-5 and 7-9) form a complex, termed the BBSome, which is essential for vesicle trafficking to cilia. Three others (BBS 6, 10 and 12) are homologous to CCT subunits and form complexes with CCT that are required for the folding of BBS2 and 7 and assembly of the BBSome. Together, these G and BBS findings point to an important role for CCT in the assembly of protein complexes that perform key physiological functions in photoreceptor cells. Thus, the underlying goal of the proposed studies is to understand at the molecular level the mechanisms of protein complex assembly by CCT in order to identify therapeutic targets to improve the folding process and treat diseases like BBS for which effective treatments are not available. Specific Aims 1 and 2 examine the structures of the G1-CCT and RGS9-G5-CCT complexes in order to understand how CCT folds both G1 and G5 and recruits RGS9 to form the G5-RGS9 dimer. Specific Aim 3 investigates the structure of the complex between the chaperonin-like BBS proteins and CCT and the mechanism of BBSome assembly. To accomplish these aims, a number of methods are proposed including site-directed mutagenesis and cell-based binding assays, pulse-chase assembly measurements of protein complexes, purification of large protein complexes and cryo-electron microscopy. A strong team of collaborators has been assembled to provide the necessary expertise to successfully execute the proposed experiments.